Paper containers for liquids are widely used as convenient containers for accommodating liquid beverages such as fruit beverages, tea, coffee, milk beverages and soup, and alcoholic drinks such as sake and shochu. Examples of the shape of the paper containers for liquids include a gable-top shape, a brick shape, and a cylindrical shape.
For example, in production of a brick-shaped container, scores for containers are formed on a packaging material, which is composed of a paper layer and a thermoplastic resin layers disposed on the front and back surfaces of the paper layer, and an edge protection tape is adhered to an end portion of the packaging material. Then, the packaging material is formed into a tubular shape, and the end portions of the packaging material are overlapped and bonded to each other to form a tubular shaped packaging material. The lower end of the tubular shaped packaging material is transversely bonded to thereby seal the lower end of the container. Subsequently, after the tubular shaped packaging material is filled with contents, a spout position (upper end) of the container is sealed and severed in a transverse direction by a liquid-pouch sealing technique. Then, the packaging material is formed into a three-dimensional shape, which is a final product shape.
The tubular packaging material is sealed in the transverse direction by using an ultrasonic sealing apparatus, for example, as disclosed in PTL 1.
That is, in the ultrasonic sealing apparatus, a sealing position of the container is pressed between an end face of a horn and an end face of an anvil, which face each other. As the horn transmits ultrasonic vibrations to the packaging material, thermoplastic resin on the surface of the packaging material is melted for seal by heat generated at the interface of the package material (at a position where surfaces of the overlapped sheet members are in contact with each other). In particular, the sealing state of the container for liquids is of importance, and the sealing described above requires a strength sufficient to protect the contents and withstand transport and drop impact.
Further, when a seal length is long, the seal length is covered by using a pair of horns (converter heads) juxtaposed as shown in FIG. 10 of PTL 2. In this case, a bonded portion of the tubular shaped packaging material, in which end portions are overlapped with each other, is positioned in a gap between the pair of horns, and the bonded portion has a relatively increased thickness. Accordingly, in the technique disclosed in PTL 2, a recess is formed on an anvil at a position corresponding to the gap between the pair of horns.